1. Field of the Invention
The present invention relates to a switching power supply apparatus in which output produced by switching D.C. input on the primary side is rectified in a rectification circuit, and is supplied to a load, and more particularly, to an energy-storage type switching power supply apparatus in which a rectification diode and a switching element in parallel to the rectification diode are connected in the rectification circuit.
2. Description of the Related Art
Conventionally, for switching power supply apparatus which can satisfy demands for high quality and can be formed at relatively low expenditure, the RCC (ringing choke converter) as disclosed in (1) Japanese Unexamined Utility Model Publication No. 63-100993 has been employed. In such ringing choke converters and flyback converters, a diode is connected to a rectification circuit on the secondary side of a transformer. That is, when a switching element on the primary side of the transformer is on, input voltage is applied to the primary winding, allowing current to flow, so that energy is stored in the transformer. When the switching element is off, the energy stored in the transformer is released from the secondary winding in the form of current. The current is rectified and smoothed to obtain an output voltage. The output voltage is stabilized by control of the on-time of the switching element.
(2) A switching power supply apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2-261053 has the following constitution. A switching means is connected in parallel to a rectification diode on the secondary side of a transformer. An output voltage obtained by rectification in the rectification circuit and smoothing is applied via the switching means to the secondary winding of the transformer, and energy is regenerated in the primary.
(3) In a switching power supply apparatus disclosed in Japanese Unexamined Patent Application Publication No. 9-271167, a synchronous rectifier comprising a MOSFET is connected to a rectification circuit on the secondary side of a transformer. When energy stored in the transformer is released in the form of current, the MOSFET is turned on, whereby the rectification loss is reduced. Moreover, the arrangement is such that the MOSFET is driven by means of a drive winding provided in the transformer.
However, the above-mentioned switching power supply apparatus of (1) to (3) have the following inconveniences.
(1) Japanese Unexamined Utility Model Publication No. 63-100993.
In the method using only the diode as the rectifying means, the forward voltage drop of the diode is large (about 0.6V). This causes the problem that a loss generated by the voltage drop reduces the efficiency and increases the temperature of the diode itself.
(2) Japanese Unexamined Patent Application Publication No. 2-261053.
In this switching power supply apparatus, rectification on the primary side of the transformer is controlled with the switching means connected in parallel to the diode, so that the output voltage is stabilized. Therefore, the magnetic flux change range is constant, irrespective of the output power, and the maximum exciting energy is stored in the transformer at all times. Thus, the conduction loss and the core loss of the transformer, and moreover, a loss caused by the regeneration of current is large, and especially, the efficiency at light-loading and non-loading is reduced. Moreover, described is an example, in which the current to be regenerated in the secondary winding is made constant, so that the magnetic flux amplitude at light-loading is decreased. In this case, it is necessary to provide a circuit for detecting current to be returned to the secondary winding, and the power loss caused by the detection circuit is problematic.
(3) Japanese Unexamined Patent Application Publication No. 9-271167.
In this switching power supply apparatus, the MOSFET, which is the synchronous rectifier, is driven only by the drive winding provided in the transformer. Accordingly, the on-time of the MOSFET is determined by the relation between the generation voltage in the drive winding and the threshold voltage at the gate of the MOSFET. In general, generation voltage in a drive winding is determined by a turns ratio, so that it is difficult for the generation voltage to have an optional value. Thus, adjustment of the on-time of the MOSFET is difficult. Moreover, the threshold voltage of MOSFET is not constant and is varied in some range in general. The on-time of the MOSFET is considerably changed, depending on the threshold voltage, and it is very difficult to determined the optimum on-time of the MOSFET. Furthermore, since the MOSFET is turned off by use of a voltage drop caused by spontaneous discharging of the gate terminal voltage of the MOSFET, the turn-off speed of the MOSFET is low, the switching loss is large, and the efficiency is low. The MOSFET is heated.
In view of the forgoing, the present invention has been devised. It is an object of the present invention to provide a switching power supply apparatus with which a high efficiency, high stabilization, and small-size and light-weight can be realized.
To solve the above-described problems, the switching power supply apparatus comprises a transformer having a primary winding and a secondary winding, a first switching element connected in series with the primary winding, a first control circuit for controlling the on-time of the first switching element whereby the output is controlled, and a rectification circuit for rectifying the output from the secondary winding, whereby input voltage is applied to the primary winding when the first switching element is on and causes current to flow therein so that energy is stored in the transformer, and the energy stored in the transformer is released as electric current from the secondary winding when the first switching element is off, and the current is rectified in the rectification circuit to obtain an output, wherein the rectification circuit comprises a rectification diode, a second switching element connected in parallel to the rectification diode, a second switching element drive winding provided in the transformer to generate a voltage which causes the second switching element to be on, and a second control circuit for turning on the second switching element with a voltage from the second switching element drive winding, and turning off the second switching element after a time determined by a predetermined time constant by use of a further switching element connected to the control terminal of the second switching element.
The switching power supply apparatus in accordance with the present invention is an RCC (ringing choke converter) type switching power supply apparatus, in which energy stored in the transformer when the first switching element is on is released in the form of current to the secondary side when the first switching element is turned off. That is, when the first switching element turns on, an input voltage is applied to the primary winding, allowing current to flow, so that energy is stored in the transformer. When the first switching element turns off, the energy stored in the transformer is released as electric current from the secondary winding. The current is rectified and smoothed to obtain an output voltage. In this case, when the first switching element is turned off, voltage is generated in the second switching element drive winding provided in the transformer, and the voltage is applied to the control terminal of the second switching element, so that the second switching element is turned on to conduct. Then, the current from the secondary winding flows through the rectification diode, and the second switching element connected in parallel to the rectification diode. However, since the second switching element is selected so that the voltage drop of the second switching element is smaller than that in the forward direction of the rectification diode, most of the output current flows in the second switching element. Accordingly, the rectification loss can be reduced.
In general, as the voltage drops of diodes, forward voltages are dominant. Even if the diode is connected in parallel, the voltage drop is scarcely changed. On the other hand, for switching elements such as MOSFETs or the like, voltage drops caused by on-resistance are dominant. Accordingly, the voltage drop can be easily decreased to be smaller than the forward voltage of the diode by parallel connection of the switching element such as a MOSFET or the like.
The switching means connected to the control terminal of the second switching element turns on after the passing of a time-period, determined by a predetermined time constant, from the time when voltage is generated in the second switching element drive winding, whereby the second switching element is turned off. At this time, the reverse voltage is applied to the second switching element and the rectification diode. The capacitive impedance equivalent from the standpoint of the rectification diode and the winding inductance of the transformer resonate with each other, and voltage is applied to the control terminal of the first switching element, so that the first switching element turns on. As described above, the first and second switching elements are alternately turned off and on so as to sandwich a time-period when both of them are off, and the on-time of the first switching element is controlled, correspondingly to the output voltage, whereby the output voltage is stabilized.
When the load is light, the output voltage is applied to the secondary winding of the transformer before passing of the time-period determined by the above predetermined time constant. After the second switching element turns off, the regeneration current flows in the input source via the primary winding of the transformer. The first switching element can be turned off, with the regenerative current, after charges at both ends of the first switching element are discharged, so that the voltage across the first switching element becomes zero. For this reason, the zero-voltage switching operation of the first switching element is enabled, so that the switching loss is reduced. Moreover, if the load becomes light, the first switching element is turned on after the second switching element is turned off after the time-period determined by the above predetermined time constant. Accordingly, the oscillation frequency can be prevented from increasing when the load is light, and thereby, intermittent oscillation or the like which contributes to deteriorated response can be prevented.
Since the rectification loss is reduced, and the switching loss is decreased as described above, a switching power supply apparatus of high efficiency can be realized. Moreover, the on-time of the second switching element is primarily determined by the predetermined time constant. Accordingly, intermittent oscillation can be prevented, in which the oscillation frequency is increased which would contribute to poor response.
According to an aspect, the second control circuit comprises a transistor connected to the control terminal of the second switching element, and an RC time constant circuit connected to the control terminal of the transistor, and is arranged so that output voltage from the second switching element drive winding is applied to the RC time constant circuit.
Since the second control circuit comprises the RC time constant circuit including the transistor, parts such as a control IC or the like are not needed. Inexpensive parts in less number can be used to form the circuit.
According to another aspect, the second control circuit contains a resistor connected between the control terminal of the second switching element and the second switching element drive winding.
A resistor which can delay the time when the second switching element is turned on is provided. Accordingly, given is the time in which charges stored at both ends in the off-period of the second switching element can be discharged. Thus, the second switching element can be zero-voltage switching operated, and the switching loss can be reduced.
According to still another aspect, the second control circuit contains a capacitive impedance connected between the control terminal of the second switching element and the second switching element drive winding.
The delay time of the above explained second control circuit can be produced by means of a resistor connected to the control terminal of the second switching element. By additional connection of the capacitive impedance in series to the resistor, the delay time can be adjusted to be optimum. Furthermore, the capacitive impedance can be singly connected for adjustment of the delay time, instead of the resistor explained above.
Further, DC current flowing from the second switching element drive winding into the control terminal of the second switching element can be cut. Thus, the drive loss can be reduced.
According to still another aspect, the second control circuit contains a time constant adjustment circuit for changing the time constant based on a signal corresponding to the magnitude of a load.
In the above-described aspect, the time constant of the second control circuit is fixed. Accordingly, the turn-off timing of the second switching element is constant, irrespective of the magnitude of the load.
Normally, if the load is light, the direction of the output current is inverted while the second switching element is on, and energy caused by the output voltage is stored in the secondary winding. When the second switching element is turned off, the energy stored in the secondary winding is released as regenerative current in the direction opposite to the ordinary one into the first switching element in the primary. When the regenerative current flows, the charging electric-charges of the parasitic capacitive impedance of the first switching element are released. The first switching element is zero-voltage switched, so that the switching loss is decreased. On the other hand, if the load is heavy, no regenerative current flows, so that a switching loss in the first switching element is generated, due to no flowing of the regenerative current. However, the regenerative energy is energy which is fed from the secondary, and is regenerated in the primary. If the regeneration amount is large, conduction loss is caused in the switching element and the transformer, due to the regenerative current. As a result, the overall efficiency deteriorates.
Accordingly, if the apparatus is configured so that the first switching element causes regeneration current to flow to such a degree that no switching loss is generated, irrespective of the magnitude of the load, the switching loss and the conduction loss can be significantly reduced. The apparatus can be made more efficient. Accordingly, in the present invention, the regeneration amount is caused even at heavy-loading by lengthening the time constant when the load is increased, and to the contrary, the regeneration amount is reduced by shortening the time constant when the load is decreased.
According to still another aspect, a capacitive impedance is connected in parallel to the first switching element or the second switching element.
By connecting the capacitive impedance in parallel to the first or second switching element, the voltage across the switching element at switching can be prevented from changing steeply. Thus, noise reduction can be realized. Moreover, especially by connecting the capacitive impedance in parallel to the second switching element, the reverse recovery loss of the rectification diode can be reduced.
According to still another aspect, a switching power supply apparatus further comprises an inductor connected in series with the primary winding, and a series circuit comprising a capacitor and a third switching element, connected in parallel to the series circuit comprising the inductor and the primary winding, whereby the first control circuit causes the first and third switching elements to turn on and off alternately, so as to sandwich a time-period when both switching elements are off, and controls the on-time of the switching elements, whereby the output is controlled.
In this embodiment, when the first switching element turns off, the second and third switching elements turn on. When the third switching element turns off, resonant current flows in the primary winding, caused by the resonance of the inductor and the capacitor. Then, the third switching element and the first switching element turn on and off, alternately, sandwiching a time-period in which both of the switching elements are off.
According to the invention, the output current has a sinusoidal wave-form in which the wave rises mildly from the zero voltage, due to the resonant current in the primary. For this reason, the current peak value can be reduced as compared with a conventional inverted triangular wave-form. If the sinusoidal wave-form and the inverted triangular wave-form are compared in the same average current level, the effective current can be reduced. Thus, the conduction loss, caused by the rectification circuit, can be decreased. Moreover, by forming a resonant wave-form, that is, designing the conduction time-period of the second switching element and a half of the resonant cycle of the resonant current to be equal to each other, much current can be supplied to the load in the conduction time-period of the second switching element, irrespective of the magnitude of the load, and on the contrary, the current flowing in the rectification diode can be reduced. Therefore, the rectification loss can be further reduced. The switching power supply apparatus using the third switching element is described in U.S. Pat. No. 6,061,252 and Japanese Unexamined Patent Publication No. 11-187664, both of which are assigned to the assignee of the present invention, and the disclosures of which are hereby incorporated by reference.
According to still another aspect, the switching power supply apparatus further comprises an inductor connected in series with the primary winding, and a series circuit comprising a capacitor and a third switching element, connected in parallel to the first switching element, whereby the first control circuit causes the first and third switching elements to turn on and off alternately, so as to sandwich a time-period when both of the switching elements are off, and controls the on-time of the switching elements, whereby the output is controlled.
Also in this embodiment, similarly to the above, the output current wave-form in the secondary can be made a sinusoidal wave-form which is mildly led from the zero current.
According to still another aspect, the switching power supply apparatus further comprises an inductor and a capacitor connected in series with the primary winding, respectively, and a third switching element connected in parallel to a series circuit comprising the inductor, the capacitor, and the primary winding, whereby the first control circuit causes the first and third switching elements to turn on and off alternately, so as to sandwich a time-period when both of the switching elements are off, and controls the on-time of the switching elements, whereby the output is controlled.
Also in this embodiment, similarly to the above, the output current wave-form in the secondary can be made a sinusoidal wave-form which is mildly led from the zero current.
According to still another aspect, the inductor is the leakage inductor of the transformer.
The number of parts can be decreased, and costs can be reduced by use of the leakage inductor of the transformer.
According to still another aspect, the first control circuit controls the third switching element so that it turns off after the second switching element turns off.
In this embodiment, control is carried out so that the third switching element turns off after the second switching element turns off. By this, energy, which is released as output from the primary, is not regenerated in the input source. By this, no energy transfer occurs between the primary and the secondary. The loss caused by the regeneration (circulation loss) can be reduced.
According to still another aspect, the first control circuit, with a bias winding provided in the transformer, controls the on-time of the first switching element based on the output of the bias winding to control the output, and oscillates autonomously.
According to still another aspect, the first control circuit, with a bias winding provided in the transformer, controls the on-time of the first switching element and the third switching element, based on the output of the bias winding to control the output, and oscillates autonomously.
By employing such an arrangement, the autonomous oscillation is enabled. An oscillation IC or the like is not needed, and the number of parts can be significantly reduced. Moreover, the first and third switching elements are driven by use of the magnetic coupling of the transformer. This easily enables a time-period when both of the two switching elements, that is, the first and second switching elements, or the first and third switching elements are off, respectively, is provided, and both of them turn on and off, alternately, sandwiching the time-period. In addition, a loss, due to short-circuit current or the like, caused by simultaneous on times, and damage to the switching element can be prevented.
According to still another aspect, the at least one of the first, second, and third switching elements is a field-effect transistor.
Since at least one of the first, second, and third switching elements comprises a field-effect transistor, the parasitic diode between the drainxe2x80x94source as a diode element, and the parasitic capacitance between the drainxe2x80x94source as the capacitive impedance can be used, respectively.
According to still another aspect, the rectification diode comprises the parasitic diode of the field-effect transistor.
Since the rectification diode comprises the parasitic diode of the field-effect transistor, it is not necessary to prepare the rectification diode of the rectification circuit as a discrete part, and the size and weight can be reduced.
For the purpose of illustrating the invention, there is shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.